The present invention relates to an image forming device such as a copier, a printer and the like by use of an electrophotographic process, an optical scanning device incorporated in the image forming device, and a holding mechanism for a long length optical element incorporated in the optical scanning device and the like.
An optical scanning device incorporated in an image forming device is structured to deflect a luminous flux emitted from a light source by an optical deflector, lead a deflected luminous flux to a scanned surface via a scanning optical system and scan on the scanning surface. The deflected luminous flux is modulated by an image signal, and then a surface of a photoreceptor which is evenly charged is scanned by a modulated luminous flux. And thereby, a latent image corresponding to the image signal is formed on the surface of the photoreceptor. This forming process of the latent image is equivalent to an exposure process of electrophotographic processes. Subsequently, via a developing process, a transferring process, and a fixing process, an image is formed on, for example, paper having a sheet shape.
The scanning optical system of the optical scanning device leads a luminous flux which is deflected with a constant angular velocity by an optical deflector to a scanned surface, and has an fθ characteristic to scan at a constant angular velocity on the scanned surface and a function to converge a luminous flux on the scanned surface. The scanning optical system, in a whole deflected range of the deflected luminous flux by the optical deflector, in order to take full advantage of the fθ characteristic and a convergence function, comprises a long length optical element which is long in a scanning direction. Hereinafter, a main scanning direction is taken as a movement direction of a deflected luminous flux by an optical deflector and a sub-scanning direction is taken as a direction which is orthogonal to the main scanning direction. The long length scanning element is long in the main scanning direction.
In recent years, plastic material has been used in an optical element of a scanning optical system. Plastic is excellent for use in mass production, but on the other hand, plastic optical elements of scanning systems having undesired shapes are often formed. This is because an uneven temperature distribution occurs in a mold when forming, a cooling process is not performed evenly after removing from the mold and so on. As for an optical element used in a scanning optical system, in particular, as for an optical element placed between an optical deflector and a scanned surface, the long length optical element described above is used in many cases. However, the long length optical element may curve in the sub-scanning direction, and a shift of a scanning position to a direction corresponding to sub-scanning such as a tilt of a scanning line, a curve of the scanning line and so on occur depending on a holding structure. In addition, an error in mounting an optical element on a housing also causes the shift of the scanning position to the direction corresponding to sub-scanning, which may often be of a size that can not be ignored.
Moreover, in an image forming device having a plurality of scanning sections and a plurality of photoreceptors corresponding to each of the plurality of scanning sections, an amount of the shift of the scanning position to the direction corresponding to sub-scanning such as the curve of the scanning line and so on per scanning section may be different due to a temperature deviation among housings which hold and fix each of the plurality of scanning sections. Also, in a system in which a plurality of light beams are emitted into a single optical deflector and scanned, and an optical element is superposed on the sub-scanning direction and placed, that is, in a system in which all scanning sections are held in a same optical housing, the shift of the scanning position to the direction corresponding to sub-scanning such as the tilt of the scanning line, the curve of the scanning line and the like on each photoreceptor per scanning section may be different due to errors in the shape and mounting of the scanning optical system and the influence of the uneven temperature distribution in the same optical housing.
In a tandem-type full-color copier, four photoreceptor drums respectively corresponding to cyan (C), magenta (M), yellow (Y) and black (K) colors are disposed in a row along a carrier surface of a transfer belt. Optical scanning devices are placed corresponding to each of the four photoreceptor drums. A luminous flux is scanned by each of the optical scanning devices, and electrostatic latent images are formed on a circumferential surface of each of the four photoreceptor drums, and these electrostatic latent images are developed by toners corresponding to colors, and a developed images are transferred sequentially on a sheet carried by the transfer belt and then multi-color images are formed. Accordingly, if the shift of the scanning position in the direction corresponding to sub-scanning occurs per the optical scanning device corresponding to each color, degradation of image quality, a color shift and the like are caused.
There have been proposed various holding mechanisms for long length optical elements in order to avoid the possibilities of problems occurring in the optical scanning device and the image forming device described above.
The invention disclosed in Japanese patent No. 3913979 is one of the above, and relates to an optical scanning device having a holding mechanism provided with a curve of a scanning line adjusting section which adjusts a curve of a scanning line on a scanned surface by forcibly deflecting an optical element held by a holding member in a sub-scanning direction. Japanese patent No. 3913979 discloses that a plurality of the adjusting sections which forcibly deflect the optical element is placed in the holding member, however, controlling of a tilt of the optical element itself in a sub-scanning cross-section is not described. Japanese patent publication No. 2001-166235 also discloses that a scanning optical device which is structured similarly to that as described above.
Japanese patent No. 3569412 discloses that an optical scanning device comprises a plastic lens which extends in a main scanning direction and a curve adjusting section which deflects forcibly the plastic lens in a sub-scanning direction, and a curve adjusting section is integrally provided with the plastic lens. However, also in Japanese patent No. 3569412, controlling of the tilt of the optical element itself in the sub-scanning cross-section is not mentioned.
Japanese patent publication No. 2006-323356 discloses a scanning optical system, an optical scanning device and an image forming device which correct a complicated curve of a scanning line (symmetric W or M shapes) to make a scanning lens deform in a twisted state in a sub-scanning cross-section by a pressing section provided in a center of the scanning lens. In the invention disclosed in Japanese patent publication No. 2006-323356, a precondition is that the curve of the scanning line which is correctable is symmetric, so that the curve of the scanning line which is not symmetric can not be corrected with high precision.
Many inventions such as Japanese patent numbers 3913979 and 3569412 and Japanese patent publication numbers 2001-166235 and 2006-323356 have been proposed to avoid the possibilities of the problems described above.
For example, Japanese patent No. 3569412 and Japanese patent publication No. 2006-323356 disclose that a shape of the scanning line of the scanned surface is corrected to make the long length optical element having a power at least in the sub-scanning direction deform in a deflected state by use of an adjusting section. However, the adjusting section having only a single structure is disclosed in the inventions above. Accordingly, it is possible to correct the curve of the scanning line which is symmetric, but it is not possible to correct the curve of the scanning line which is not symmetric with high precision.
On the other hand, it is possible for the inventions disclosed in Japanese patent No. 3913979 and Japanese patent publication No. 2001-166235, which have a plurality of adjusting sections, to correct the curve of the scanning line which is not symmetric. However, Japanese patent No. Japanese patent No. 3913979 and Japanese patent publication No. 2001-166235, do not disclose that occurrences of a tilt distribution in a longitudinal direction in the sub-scanning cross-section of long length optical elements such as a scanning lens, a scanning mirror, a cylindrical lens and the like (this is called “a twisted deformation”) are controlled. The adjusting section of the long length optical element disclosed in Japanese patent No. 36913979 and Japanese patent publication No. 2001-166235 adopts a structure such that “a pressing section” such as an adjusting screw and so on presses one of an upper surface or a lower surface of the long length optical element and “a spring section” such as a flat spring, a compression spring and so on which opposes the pressing force act on another surface of the long length optical element. The adjusting section as described above is provided and deforms the long length optical elements in a deflected manner, so that the shape of the scanning line on the scanned surface can be corrected.
However, in a case where long length optical elements are those having a power at least in a main scanning direction, such as a scanning lens, a scanning mirror, and so on, a cross-section in the longitudinal direction is not always uniform. Therefore, there is a possibility of a twisted deformation occurring (the tilt distribution in the longitudinal direction in the sub-scanning cross-section) by an arrangement of the pressing section and the spring section. Compared with a case where only a deflected deformation occurs, in a case where the twisted deformation occurs, there is another possibility that a desired adjustment of the curve of the scanning line is not performed, because a curve of the scanning line is generated by the twisted deformation. In a case where an optical system from a deflector to a scanned surface is a magnifying optical system, an amount of the deflected deformation of the long length optical element is increased, and an amount of the curve of the scanning line on the scanned surface becomes large. For example, in a case where an adjusting screw with a 0.36 mm screw pitch is used as the pressing section in the adjusting section, an advancing distance is 10 μm when the adjusting screw rotates by 10 degrees. That is, the long length optical element deforms by 10 μm in the deflected manner, but on the other hand, in a case of the magnifying optical system, the amount of the deflected deformation (10 μm) is increased and a large curve of the scanning line occurs. That is to say, a resulting adjustment amount becomes large comparing to an adjustment amount of the adjusting screw. Here, when the deflected deformation and the (unintended) twisted deformation occur, a larger curve of the scanning line occurs, that is, an adjustment sensitivity becomes too high, and there is still another possibility that it is difficult to adjust with high precision using adjustment tools without a speed reducing mechanism such as an ordinary screwdriver and so on.
“The twisted deformation” described above, as illustrated in FIG. 14A, is a deformation when a rotational moment around a rotational axis (i.e. torque) which is approximately parallel to the longitudinal direction works, and it is also called “a torsional deformation”. “The deflected deformation” described above, as illustrated in FIG. 14B, is a deformation when a shearing force in an orthogonal direction to the longitudinal direction works.